The present invention relates to a pressure transmitter. In particular, it relates to an isolation diaphragm in a pressure transmitter.
Pressure transmitters have a pressure sensor coupled to an isolation diaphragm. The isolation diaphragm isolates the pressure sensor from corrosive process fluids being sensed. Pressure is transferred from the isolation diaphragm to the sensor through a substantially incompressible isolation fluid carried in a passageway. U.S. Pat. No. 4,833,922 entitled MODULAR PRESSURE TRANSMITTER and U.S. Pat. No. 5,094,109 entitled PRESSURE TRANSMITTER WITH STRESS ISOLATION DEPRESSION show pressure transmitters of this type.
Since the isolation diaphragm is exposed to process fluid, it is subjected to corrosive materials which may damage its structure. Sapphire and ceramic materials are well suited for isolation diaphragms because of their ruggedness and corrosion resistance. However, stresses applied to seal a diaphragm made of these materials tend to change the diaphragm's stiffness or shape and cause errors. Furthermore, diaphragms made of these materials have not been able to displace a sufficient quantity of fluid to obtain accurate pressure measurements. A practical seal that is not susceptible to corrosion has not been available.